The present invention relates to a method for producing a fibrous sheet and more particularly it relates to so-called internal application of latex in production of a fibrous sheet by known wet paper making techniques or wet non-woven fabric making techniques which comprises previously flocculating a carboxyl-modified anionic latex, adding the resultant flocculate to a fiber slurry, making the slurry into a sheet and drying the sheet to obtain a fibrous sheet. The method for the internal application of latex can be roughly classified into the following two techniques.
(1) A method which comprises adding an anionic latex to a fiber slurry and flocculating the latex with a polyvalent metal salt to deposit the latex onto the fibers. This is called beater addition.
(A) Direct method: This method comprises said steps in the order as mentioned above.
(B) Reverse method (Armstrong method in another name): This method comprises adding a polyvalent metal salt to a fiber slurry to adsorb the salt onto the fibers and then adding an anionic latex thereto to flocculate and deposite the latex onto the fibers.
(C) Bardac Process: The order of addition is the same as that of said reverse method (b), but a melamine resin acid colloid is used in place of the polyvalent metal salt.
(2) A method which comprises previously flocculating a latex, dispersing the resultant flocculate in a fiber slurry and retaining the latex by filtering action in paper making process.
The present invention provides an improved method which belongs to the technical field of the above mentioned (2). The technique (1) has been developed for the purpose of production of special papers which uses pulps as a starting material, but this method has various defects. That is, this method can be relatively smoothly carried out in the case of adding a latex in an amount of less than 10% by weight of fibers, but when the latex is added in an amount of more than 15% by weight, the following disadvantages occur: Uniform flocculation and deposition of the latex onto fibers difficultly occur and bulk flocculates of more than 1 mm separated from fibers are present to result in troubles in paper making procedure. Furthermore, the resultant fibrous sheet which is merely filled with latex masses has low strength, has resin specks throughout the surface to lose the value as a product and lacks water resistance, high flexibility, and softness which are characteristics of a sheet to which latex is added. Furthermore, this method is not effective for production of non-woven fabrics mainly composed of regenerated fibers or synthetic fibers and substantially no strength is developed. The reasons therefor are considered to be as follows: The regenerated fibers and synthetic fibers have smaller surface area than beaten pulp and so amount of deposition of latex is very small. Furthermore, even if latex can be uniformly deposited onto the surface of fibers by some special means, being different from papers and boards which are mainly composed of beaten pulp and have a high density, non-woven fabrics which are mainly composed of regenerated fibers or synthetic fibers and which have a low density and must have a bulkiness contain extremely small number of bonding points of fibers and hence efficiency of latex is markedly decreased.
Intermediate techniques between the methods (1) and (2) are proposed in Japanese Pat. Publications Nos. 75/64 and 14272/65. According to these methods which attempt to remove the defects in the method (1), pre-flocculated particles of more than 1.mu. and less than 60-100.mu. are previously prepared, these particles are added to a fiber slurry adjusted to neutral and then pH of the slurry is adjusted to acidic to cause final flocculation of said pre-flocculated particles and deposition of the particles onto the fibers. However, coarse masses of more than 1 mm are formed when pH is reduced and free masses of latex are present to result in troubles in paper making and formation of specks of latex on the resultant fibrous sheet. Furthermore, tackiness of the fibrous sheet is increased and blocking is apt to occur at rewinding of the roll of the sheet to cause fluffing.
Examples of the method (2) which is based on the same idea as addition of fibrous binder or powdery binder to a fiber slurry are disclosed in Japanese Pat. Publication No. 20195/73 and U.S. Pat. No. 3,776,812.
Japanese Pat. Publication No. 20195/73 discloses that a latex liquid is thickened with carboxymethyl cellulose or polyacrylic ester and then the latex is flocculated with an aluminum salt to obtain flocculate of uniform size. However, according to this method, thickening agent is also flocculated and tackiness is lost at flocculation of latex. Thus, effect of the thickening agent to obtain uniform flocculated particles cannot be expected. Moreover, coarse masses of more than 1 mm are apt to be produced due to the shock of addition which is caused by intensity of electric charge of the aluminum ion and reduction in pH. Therefore, the troubles in paper making as in the method (1) tend to occur and moreover strength of the resultant web is not satisfactory. Such method is considered effective only when this is combined with a special coagulating apparatus. This method has the further defect that the wet strength of the resultant sheet is low because a large amount of aluminum salt is incorporated into the latex flocculate.
U.S. Pat. No. 3,776,812 proposes the method which comprises producing fine pre-flocculates of less than 50.mu. by mixing carboxyl modified latex with water soluble cationic polymer at a pH of 6-9 and adjusting the pH to 2-4 to obtain fibrous flocculate (particle size 100-500.mu.). However, detailed examination of this method shows that even when the pH of the pre-flocculate is reduced to 2-4, the resultant flocculated particles have 50-100.mu. at largest. In this case, retention of the flocculate at paper making process is low and properties of the resultant sheet are also not satisfactory.
According to the experiments by the inventors, it has been found that it is essential that particle size of the flocculate should be adjusted to 100-500.mu., preferably 100-300.mu..
Particle size of the flocculate is defined as maximum diameter of each flocculate when observed under microscope.
As mentioned above, the method (2) has stil many defects, which are summarized as follows:
(a) Preparation of flocculate having uniform particle size is very difficult. That is, when too fine, the retention of the flocculate at paper making is decreased and when too coarse, bonding joints of the fibers are conspicuously decreased to cause reduction in strength and moreover specks are apt to occur on the surface of the resultant sheet.
(b) There occur many troubles on paper machine, for example, clogging of wire, stain of felt, sticking of the sheet to drier, etc.
(c) Tackiness of flocculated latex is great and the resultant sheet after drying step still has tackiness. Thus, the rolled fibrous sheets are liable to adhere to each other to result in fluffing at rewinding of the roll of the sheet.
(d) A special solidifying apparatus is necessary.